The U.S. economic expansion is entering its sixth year with the best stretch of job growth in almost a decade. Beneath the shiny exterior, however, lurk soft patches that worry economists and policy makers.

Investors on Thursday seized on robust jobs numbers—not lingering soft spots—and drove the Dow Jones Industrial Average past 17000 for the first time. The Dow closed up 92.02 points, or 0.5%, at 17069.26 in holiday-shortened...